Thermocouple structure and method of manufacturing same

ABSTRACT

A thermocouple structure useful for high temperature applications comprises a pair of thermocouple wires joined at one end to form a thermocouple, the wires being enclosed within a metallic sheath which is open at one end. The thermocouple and the wires within the sheath are surrounded and insulated by crushed compacted magnesium oxide. The end of the sheath from which the thermocouple wires extend is filled with ceramic cement to seal the crushed magnesium oxide and the wires therein. The thermocouple structure is prepared by inserting stripped thermocouple wires into a cylindrical two-hole perforated magnesium oxide preform and the resulting assembly placed within a metal sheath. The resulting assembly is then swaged or reduced in diameter to crush and compact the magnesium oxide therein. The resulting swaged assembly is opened, if desired, to expose the ends of the wires which are then welded to form a thermocouple. High temperature ceramic cement is then employed to insulate the thermocouple and that end of the sheath welded shut. Ceramic cement is placed in the other end of the sheath to seal the crushed magnesium oxide and wires therein.

This invention relates to thermocouples and thermocouple structures and devices containing a thermocouple. In one embodiment this invention relates to a thermocouple structure suitable for high temperature applications, i.e. for the sensing or measuring of high temperatures, e.g. above about 600°F. In another embodiment this invention relates to an improved method for the manufacture of thermocouples and thermocouple structures.

Thermocouples and thermocouple structures suitable for high temperature application should be rugged and substantially trouble-free since usually such thermocouples are positioned in difficult-to-reach locations and oftentimes at such locations are subjected to not only the high temperatures to be measured but also to vibration and erosive and corrosive environmental conditions.

It is an object of this invention to provide a thermocouple structure which is capable of installation and use under demanding conditions.

It is another object of this invention to provide a thermocouple structure capable of being manufactured at relatively low cost, particularly as compared with other comparable known thermocouple structures.

It is still another object of this invention to provide an improved process for the manufacture of thermocouple structures.

In at least one embodiment of the practices of this invention at least one of the foregoing objects will be achieved.

How these and other objects of this invention are achieved will become apparent in the light of the accompanying disclosure made with reference to the accompanying drawings which illustrate the various components and steps in the manufacture of thermocouple structures in accordance with this invention and wherein

FIG. 1 is a cross-sectional view of a metallic sheath employed in the manufacture of the finished thermocouple structure;

FIG. 2 is a cross-sectional view of a two-hole, perforated mineral oxide (MgO) preform employed in the manufacture of the finished thermocouple structure;

FIG. 3 is a section of a length of thermocouple wires useful in the manufacture of the thermocouple structure;

FIG. 4 illustrates the thermocouple wires of FIG. 3 inserted in the mineral oxide preform of FIG. 2;

FIG. 5 illustrates in partial cross section the assembly comprising the mineral oxide preform-thermocouple wires assembly of FIG. 4 inserted within the sheath of FIG. 1;

FIG. 6 illustrates a size reduction or swaging operation wherein the assembly of FIG. 5 is reduced in diameter;

FIG. 7 illustrates in partial cross-section a finished grounded thermocouple in accordance with this invention;

FIG. 8 illustrates in partial cross-section a finished ungrounded thermocouple structure in accordance with this invention; and wherein

FIG. 9 schematically represents an EMF or voltage sensing device embodying a thermocouple structure in accordance with this invention.

A thermocouple structure in accordance with this invention comprises a pair of thermocouple wires joined together at one end to form a thermocouple and enclosed within a metallic sheath. The thermocouple wires including the thermocouple enclosed by the metallic sheath are insulated from the metallic sheath and the wires from each other by crushed compacted refractory electrically insulating material, such as a mineral oxide, e.g. magnesium oxide, beryllium oxide, aluminum oxide, zinc oxide, thorium oxide and the like. The other ends or portions of the thermocouple wires which may be insulated, such as by means of conventional rubber or plastic insulating material, but preferably with high temperature insulating material which may also be a plastic, such as a polyimide, e.g. Kapton, extend from the open end of the metallic sheath opposite from the closed end thereof in proximity to the thermocouple. The thermocouple is encased or enclosed within the metallic sheath and ceramic cement is added to the other open end of the metallic sheath from which the thermocouple wires extend so as to not only seal the thermocouple wires therein but also to contain and seal the crushed, compacted mineral oxide refractory material within the sheath.

Reference is now made to the drawings which illustrate components and steps in the manufacture of thermocouple structures in accordance with this invention and various embodiments of thermocouple structures and a device or apparatus employing the same.

FIG. 1 illustrates a generally cylindrical open ended metallic sheath or cylinder 10. FIG. 2 illustrates in cross-section a two-hole perforated cylindrical solid refractory mineral oxide, e.g. magnesium oxide, preform 11, the holes or perforations therethrough being identified by reference numerals 11a and 11b. FIG. 3 illustrates a pair of thermocouple wires generally indicated by reference numeral 12. Lengths 12a and 12b of the thermocouple wires are stripped of insulation which is indicated by reference numerals 14a, 14b and 15.

The stripped lengths of thermocouple wires 12a and 12b, which lengths are longer than preform 11, are inserted into preform 11 within perforations 11a and 11b, respectively, as indicated in FIG. 4, and the resulting assembly is inserted within metallic sheath 10 as illustrated in FIG. 5. Thereupon, as illustrated in FIG. 6, the assembly of FIG. 5 is subjected to a swaging or size reducing operation wherein sheath 10 containing the magnesium oxide preform 11 is reduced in diameter and the magnesium oxide material making up preform 11 crushed and compacted. The reduction is carried out to the extend as may be required by the sensor or thermocouple sensitivity desired.

As illustrated in FIG. 6 the end of the zone of migration of the crushed compacted refractory material, i.e. the magnesium oxide, making up preform 11 extends to about line AA.

If a grounded thermocouple structure is to be prepared, as illustrated in FIG. 7, the resulting structure of FIG. 6 would be welded along line BB or cut and welded between lines BB and AA so that sheath 10 would be welded to thermocouple wires 12a and 12b to provide a unitary structure therewith and to form the grounded thermocouple 12c, as fully illustrated in FIG. 7. Cement, such as potting cement 11c, would be provided at the other end of the thermocouple structure to retain the crushed compacted refractory material within sheath 10 and to firmly set thermocouple wire 15 within sheath 10.

If an ungrounded thermocouple structure is to be prepared the swaged reduced assembly illustrated in FIG. 6 would be cut along line CC. The exposed crushed compacted magnesium oxide would be removed, such as by jet abrasion, i.e. directing a jet of abrasive finely divided material onto the exposed crushed magnesium oxide preform material to remove a small amount of this material surrounding the exposed ends of thermocouple wires 12a and 12b. The ends of wires 12a and 12b are then joined together or welded together as illustrated in FIG. 8 to form ungrounded thermocouple 12d. High temperature ceramic cement 11d would then be added to surround thermocouple 12d and the end of sheath 10 would be closed at 10b, such as by joining and welding. The open end of sheath 10 would be filled with a suitable cement, such as potting cement 11c, to seal the insulated thermocouple wires therein and to maintain the crushed compacted magnesium oxide preform material within sheath 10.

Still another embodiment of the practices of this invention is illustrated in accompanying FIG. 9 wherein a thermocouple structure, generally indicated by reference numeral 16, such as may be prepared in the manner described hereinabove, is electrically connected through insulated conductors 18a and 18b which are extensions of insulated wires 12a and 12b, respectively, to a voltage measuring device 19. The apparatus of FIG. 9 might be employed to determine the temperature to which the thermocouple located at about end 10b of thermocouple structure 16 is exposed and, as indicated, depending upon the lengths of wires 18a and 18b the thermocouple may be located a substantial distance, many feet, from device 19.

In the manufacture of the thermocouple structures in accordance with the invention substantially any combination of thermocouple wires useful for measurement of high temperatures, e.g. above 500°-600°C., may be employed, such as wires made of Chromel, a nickel chromium alloy containing about 90% by weight nickel and about 10% by weight chromium, Alumel, a nickel alloy containing about 94% by weight nickel and small amounts of aluminum, manganese and silicon and constantan, a nickel copper alloy comprising about 40-45% by weight nickel and about 55-60% by weight copper.

The metallic sheath making up the thermocouple structure in accordance with this invention may be made of any suitable metal or metallic material. Preferably, the metallic sheath is made up of a stainless steel, such as Inconel, a nickel and chromium alloy, or a Haynes alloy, such as the various Hastelloys and the various Haynes Stellites. The metallic sheath is made up of a metal which is resistant to oxidative and reduced atmospheres, is corrosive resistant and is capable of withstanding relatively high temperatures. Since the purpose of the metallic sheath is to protect the thermocouple from the environment within which the thermocouple is positioned, the material making up the protective metallic sheath must be effective for this purpose.

The thermocouple structures prepared in accordance with this invention can be produced in small size and at a reduced cost. Moreover, thermocouple structures in accordance with this invention eliminate the electrical connections heretofore found necessary in conventional thermocouple structures between the thermocouple wires and conductors therefrom since in the manufacture of a thermocouple structure in the manner described herein and in accordance with this invention the thermocouple wires can lead directly from the thermocouple end of the thermocouple structure directly to the voltage sensing instrument which might be located a substantial distance from the thermocouple. There is no need for an intermediate electrical connection between the thermocouple and the thermocouple wires making up the thermocouple and the voltage sensing or temperature sensing device. In effect, in thermocouple structures prepared in accordance with this invention the thermocouple wires lead directly without any intermediate electrical connection between the thermocouple made up by joining the wires together and the voltage sensing instrument.

As will be apparent to those skilled in the art in the light of the foregoing disclosure, many modifications, alterations and substitutions are possible in the practice of this invention without departing from the spirit or scope thereof. 

I claim:
 1. A method of preparing a thermocouple structure which comprises providing a length of a pair of thermocouple wires stripped of insulating material along a portion thereof, inserting the stripped portion of said pair of wires into corresponding perforations in a crushable mineral oxide preform having a length shorter than the length of the stripped portion of said pair of wires, inserting the resulting assembly into an open-ended metallic sheath, reducing the diameter of that portion of the metallic sheath enclosing said preform such that the diameter of said sheath at one end is smaller than the diameter of said sheath at the other end, the crushable mineral oxide preform as a result of said size reduction operation being crushed and compacted around and along the length of the stripped wires within the corresponding portion of said sheath having undergone size reduction, cutting the reduced one end of said sheath to expose the ends of said thermocouple wires, removing the crushed compacted mineral oxide material surrounding the exposed ends of said thermocouple wires, welding the ends of said thermocouple wires to form a thermocouple, applying ceramic cement to insulate said thermocouple from said sheath and closing the reduced one end of said sheath to enclose the resulting insulated thermocouple.
 2. A method in accordance with claim 1 wherein the reduced one end of said sheath is welded to said thermocouple wires to close that end of the sheath and to form a grounded thermocouple.
 3. A method in accordance with claim 1 wherein said mineral oxide preform is a magnesium oxide preform.
 4. A method in accordance with claim 1 wherein said mineral oxide of said mineral oxide preform is selected from the group consisting of magnesium oxide, zirconium oxide, thorium oxide, beryllium oxide and aluminum oxide.
 5. A method in accordance with claim 1 wherein said metallic sheath is made up of a corrosion resistant stainless steel.
 6. A method in accordance with claim 1 wherein the crushed compacted mineral oxide material is removed by directing a jet of particulate abrasive material onto the exposed ends of said thermocouple wires and the surrounding crushed compacted mineral oxide to erode and remove said mineral oxide.
 7. A method in accordance with claim 1 wherein during reduction of the diameter of a portion of said metallic sheath the diameter of the thermocouple wires surrounded by a portion of the metallic sheath undergoing size reduction is also reduced.
 8. A method of preparing an EMF or voltage sensing device which comprises providing a length of a pair of thermocouple wires stripped of insulating material along a portion thereof, inserting the stripped portion of said pair of wires into corresponding perforations in a crushable mineral oxide preform having a length shorter than the length of the stripped portion of said pair of wires, inserting the resulting assembly into an open-ended metallic sheath, reducing the diameter of that portion of the metallic sheath enclosing said preform such that the diameter of said sheath at one end is smaller than the diameter of said sheath at the other end, the crushable mineral oxide preform as a result of said size reduction operation being crushed and compacted around and along the length of the stripped wires within the corresponding portion of said sheath having undergone size reduction, cutting the reduced one end of said sheath to expose the ends of said thermocouple wires, removing the crushed compacted mineral oxide material surrounding the exposed ends of said thermocouple wires, welding the ends of said thermocouple wires to form a thermocouple, applying a ceramic cement to insulate said thermocouple from said sheath, closing the reduced one end of said sheath to enclose the resulting insulated thermocouple, introducing cement into the other open end of said sheath to seal the thermocouple wires therein and electrically connecting the other ends of said pair of thermocouple wires to a voltage difference sensing means. 